1. Field of the Invention
The present invention relates to a radio-frequency device and wireless communication device, and more specifically, to a radio-frequency device and wireless communication device capable of automatically adjusting output power and radiating frequency.
2. Description of the Prior Art
A wireless communication device exchanges radio-frequency signals through an antenna to access information within a wireless communication system. A radio-frequency (RF) signal is a sinusoidal wave with a high oscillating frequency, and governments in the world have defined safety limits, e.g. by electromagnetic standards, for exposure to RF energy produced from wireless communication devices, which mainly exposes to human head or limb. The electromagnetic standards as to the RF energy exposure are based on SAR (specific absorption rate) instead of on the ration of maximum/minimum output power. SAR is a measure of the rate at which energy is absorbed by a human body when exposed to an RF electromagnetic field. According to ICNIRP (International Commission on Non-Ionizing Radiation Protection), a recommended SAR value should not exceed 2.0 W/Kg. According to FCC (Federal Communications Commission), the recommended SAR value should not exceed 1.6 W/Kg.
However, as well known to those skilled in the art, the greater antenna gain, the worse SAR value; the smaller antenna size, the narrower radiating bandwidth. A traditional method for reaching good SAR value is to dispose proximity sensor beside the antenna for detecting approaching status of the human body. In other words, when the proximity sensor detects the human body within a specific range, the wireless communication device decreases the power of the RF signal; once the human body is not detected within the specific range, the power of the RF signal is maintained or increased.
On the other hand, due to a trend of light and compact wireless communication device and growing wireless communication demands, an ideal antenna inside the wireless communication device should be small, antenna gain thereof should be high and radiating bandwidth thereof should be as wider as possible. However, as well known in the art, the antenna requires a longer current route to induce the RF signal with lower frequency. Besides, to reach multiple radiating frequency bands in the lower frequency requires much larger antenna space. To meet these requirements, additional antennas may be required for operating in another frequency band.
As a result, the additional proximity sensors and antennas for covering other frequency bands both increase design and production cost and increase complexity of the material or part management. Thus, how to solve the tradeoff between SAR and antenna performance and the tradeoff between antenna size and radiating band width have become a goal in the wireless communication industry.